1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus such as an inkjet recording apparatus that forms images on a recording medium by using an ejection head in which a plurality of liquid droplet ejection ports (nozzles) are arranged two-dimensionally at high density.
2. Description of the Related Art
An inkjet recording apparatus forms images by means of ink dots, by causing ink to be ejected from a recording head (ejection head) having nozzles for ejecting ink, in accordance with a print signal, thereby depositing ink droplets on a recording medium, such as recording paper, while moving the recording medium relatively with respect to the recording head.
In order to achieve high-resolution printing of photographic image quality, it has been sought to arrange the nozzles at high density. In relation to this, Japanese Patent Application Publication No. 2001-334661 discloses technology for achieving high density of nozzles by arranging square or rhombus shaped pressure chambers corresponding to the nozzles in a two-dimensional matrix configuration.
However, if the nozzles are arranged at high density by using the technology disclosed in Japanese Patent Application Publication No. 2001-334661 and a full line type recording head having nozzle rows extending along a length corresponding to the entire printable width of the recording medium is composed, then non-uniformity of density may occur in the image of the print result, due to differences of coalescence degree of the liquid droplets on the recording medium resulting from differences in the droplet ejection time intervals between adjacent dots. This phenomenon and the causes thereof are described with reference to FIGS. 14A to 16C.
FIG. 14A is a schematic view showing an example of a nozzle arrangement in a conventional full line type inkjet head (hereinafter referred to as “head”). FIG. 14B is a schematic view showing a dot arrangement in a solid image formed by droplets ejected from this head. Although shown schematically in FIGS. 14A and 14B, this head 200 has a length corresponding to the full width of the recording medium (not shown), and is fixed in position so as to extend in a direction (the direction indicated by the arrow M in the drawings; namely, main scanning direction) that is substantially perpendicular to the direction of conveyance of the recording medium (the direction indicated by the arrow S in the drawings; namely, sub-scanning direction).
The nozzles A-i and B-i (i=1, 2, 3, . . . , 6) forming the ink droplet ejection ports are arranged in a two-dimensional matrix configuration. More specifically, the nozzles A-i and B-i (i=1, 2, 3, . . . , 6) are arranged in a row direction aligned with the direction indicated by the arrow M that is perpendicular to the conveyance direction of the recording medium indicated by the arrow S, and in an oblique column direction that has a prescribed non-perpendicular angle θ with respect to the row direction.
When the nozzles A-i and B-i (i=1, 2, 3, . . . , 6) arranged in a matrix array as shown in FIG. 14A are driven, one line (a line formed of a row of dots or a line formed of a plurality of rows of dots) is printed in the direction perpendicular to the conveyance direction of the recording medium, by driving the nozzles (in other words, causing the nozzles to eject ink) sequentially from one end toward the other end in each of nozzle blocks. Each nozzle block is based on a unit formed by a group of nozzles aligned in the oblique column direction. Driving the nozzles in this way is defined as main scanning.
More specifically, taking the nozzles A-1, A-2, A-3, A-4, A-5 and A-6 in FIG. 14A to be one block (and taking nozzles B-1, . . . , B-6 to be another block, and so on), one line is printed in the breadthways direction of the recording medium by sequentially driving the nozzles A-1, A-2, . . . , A-6 in accordance with the conveyance speed of the recording medium.
On the other hand, “sub-scanning” is defined as to repeatedly perform printing of one line formed by the aforementioned main scanning, while the full-line head 200 and the recording medium are moved relatively to each other.
By ejecting droplets to form a solid image by means of the head 200 having the nozzle arrangement shown in FIG. 14A, the dot arrangement shown in FIG. 14B is obtained. However, a difference arises between the landing times of the droplets forming the dots that are adjacent to each other in the main scanning direction (the direction indicated by the arrow M).
More specifically, droplets are ejected to form a dot row aligned in the direction perpendicular to the direction of conveyance of the recording medium, by main scan driving concerning the nozzles A-i and B-i (i=1, 2, 3, . . . , 6), in the sequence of the dot numbers 1, 2, 3, 4, 5, 6 in FIG. 14B. The droplets land onto the print medium in the same sequence of the dot numbers 1, 2, 3, 4, 5, 6 as mentioned above.
Taking the nozzle pitch in the main scanning direction in the head 200 to be L1, the nozzle pitch in the sub-scanning direction in the head 200 to be L2, and the conveyance speed of the recording medium to be U (m/s), then the difference in droplet ejection times between adjacent nozzles that eject droplets to form adjacent dots aligned in the main scanning direction (in other words, the difference between the landing times of the droplets forming the adjacent dots) will be L2/U. However, the difference in droplet ejection times at the return section of a nozzle block, in other words, the difference in droplet ejection times concerning dot “1” formed by a droplet ejected from the nozzle B-1 and dot “6” formed by a droplet ejected from the nozzle A-6 will be L3/U (in the example of FIG. 14A, L3=5×L2).
If droplets are ejected at very high speed, then the droplet ejection time interval L2/U becomes shorter than the fixing time of the droplets (namely, the time required for drying, permeation, solidification, and the like), and therefore coalescence of the droplets occurs on the recording medium. As shown in FIG. 15, while a droplet 221 that is deposited on the recording medium 216 to form a previous dot is not completely fixed (in a state where liquid ink is still present on the surface of the recording medium 216), if a droplet 222 is deposited to form a subsequent dot, then the subsequently deposited droplet 222 is attracted toward the previously deposited droplet 221 by surface tension, and the droplet 222 then unites with the droplet 221.
The droplet coalescence phenomenon described above occurs successively as the nozzles are driven in the main scanning action, and a similar coalescence phenomenon also occurs in the sub-scanning direction. Focusing on the liquid droplet ejected from a nozzle (for example, A-6) in the last row of the nozzle block described in FIG. 14A, the droplet forming the dot number “6” in FIG. 14B ejected from the nozzle A-6 makes contact with both a droplet forming the dot number “5” ejected from the nozzle A-5 and a droplet forming the dot number “1” ejected from the nozzle B-1. Since the landing time of the droplet forming the dot number “1” ejected from the nozzle B-1 is earlier than the landing time of the droplet forming the dot number “5” ejected from the nozzle A-5 (and droplet ejection timing of the nozzle B-1 is the same as that of the nozzle A-1), then the droplet forming the dot number “1” ejected from the nozzle B-1 is more fixed than the droplet forming the dot number “5” ejected from the nozzle A-5. Consequently, the droplet forming the dot number “6” ejected from the nozzle A-6 is attracted toward the droplet forming the dot number “5” ejected from the nozzle A-5 that is adjacent to the nozzle A-6 on the left-hand side, because the time difference between the ejecting time of the nozzle A-6 and the ejecting time of the nozzle A-5 is smaller than the time difference between the ejecting time of the nozzle A-6 and the ejecting time of the nozzle B-1.
FIG. 16A is a schematic view of an ideal dot arrangement in solid printing (the target landing positions in the drive control sequence). FIG. 16B is a schematic view showing the dot positions after the droplets have moved on the recording medium due to the aforementioned droplet coalescence phenomenon. FIG. 16C is a drawing showing a schematic view of the results of coalescence of a group of dots in the same column in the paper conveyance direction (the sub-scanning direction).
As shown in FIG. 16B, the distance wd′ between the dots (dot numbers “6” and “1”) formed by droplets ejected from the nozzles A-6 and B-1 is greater than the distances between the adjacent dots formed by droplets ejected from the other nozzles A-1 to A-6. Hence, a portion of lower density compared to the other portions is created in a position on the recording medium corresponding to the region between the nozzles A-6 and B-1.
Furthermore, if sub-scanning is performed along with conveying the recording medium, then the aforementioned phenomenon is similarly repeated in the sub-scanning direction. As a result of that, stripe-shaped unevenness having lower density such as that shown in FIG. 16C, occurs in a position corresponding to the region between the nozzles A-6 and B-1 (the so-called “return section” of the matrix head). The spatial repetition cycle of this stripe-shaped unevenness corresponds to the repetition cycle of the one column block that extends in the oblique column direction having an angle θ in the two-dimensional arrangement of nozzles A-i and B-i (i=1, 2, 3, . . . , 6) shown in FIG. 14A (the distance between the nozzle A-1 and the nozzle B-1, or the pitch of the nozzle blocks in the column direction).
The higher the dot density is, the more significant the degree of the coalescence is. Hence, stripe-shaped banding (unevenness) is a particular problem in the high-density regions.
A similar problem arises in the joint sections (the positions indicated by the arrows A) of a line head shown in FIG. 17, which is formed to a long length by joining together a plurality of short heads 230.
Japanese Patent Application Publication No. 2003-34020 discloses a composition for forming one long head by combining a plurality of relatively short inkjet heads, in which the distance between nozzles at joint sections are made shorter than the normal nozzle pitch, in such a manner that the occurrence of stripe-shaped non-uniformity in density is avoided at positions of the image corresponding to the joint sections between the short heads.
The method disclosed in Japanese Patent Application Publication No. 2003-34020 is effective in reducing unevenness in high-density image regions. However, in low-density regions, the degree of coalescence of the droplets is low, or alternatively, no coalescence occurs at all. Hence, as shown in FIGS. 18A and 18B, the dot density in the region of reduced nozzle pitch (the density in the regions indicated by the arrow B and the arrow B′ in the drawings), is higher than that of the surrounding regions, and there is a problem in that this may appear adversely as a high-density stripe (black stripe). Japanese Patent Application Publication No. 2003-34020 does not refer to the problem of low-density regions.
Furthermore, the degree of coalescence of the liquid droplets on the recording medium varies depending on conditions such as the speed of the ejection, the type of recording medium, the properties of the ink, and therefore, the optimal nozzle-to-nozzle pitch varies also. If a long head is formed by joining together short heads, then it is possible to respond to change in conditions by applying a pitch adjustment function. However, in the case of an integrated matrix head, it is difficult to adjust the nozzle-to-nozzle pitch in accordance with change in the conditions.